In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplished such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist material, thickness uniformity and lack of defects in the photoresist material is a significant factor in achieving desired critical dimensions. Contaminant particles residing or introduced into the chamber where the photoresist is applied are a major cause of defects in wafers. In some cases, the contaminant particles can have thicknesses as large as the thicknesses of the wafers making the wafers virtually useless.
A typical photoresist coating process involves spinning on photoresist materal onto a wafer to form a thin film of photoresist. Some of the excess photoresist spun off from the wafer becomes an aerosol. Aerosol not removed by a cleaning system (e.g., vacuum system) may contribute to contaminating the thin photoresist film on the wafer.
In view of the above, a system/method is needed, for measuring and/or controlling contaminants particles contained in an aerosol used or formed in a lithographic process.